HVDC power transmission is becoming increasingly important due to the steadily rising need for bulk power delivery and interconnected power transmission and distribution systems. An HVDC grid typically comprises multiple terminals for converting an alternating current (AC) power source, using, e.g., voltage source converters (VSCs), for transmission over HVDC transmission lines. Within the grid, a terminal may be connected to multiple terminals resulting in different types of topologies. Such a multiple terminal grid enables efficient congestion management and has an improved stability against disturbances.
Typically, transmission lines which are solely based on cables suffer mostly from permanent line faults, whereas transmission lines comprising overhead lines may suffer both from permanent and temporary line faults. In particular, temporary faults due to lightning are much more frequent than permanent line faults, making a restart, after the fault is cleared, desirable.
In the case of symmetric monopole transmission lines, comprising two poles, a voltage unbalance may occur in the event of an earth fault on one of the poles. In order to facilitate automatic restarts in the event of temporary earths faults, it is desirable to remove the voltage unbalance. The voltage unbalance may, e.g., be removed utilizing discharge resistors in combination with DC choppers.